1. Field of the Invention
The present invention relates to an optical module employing a laser diode device, and more particularly to an optical module used for high speed optical communications, which require impedance matching.
2. Background Art
In recent years, there has been a need to reduce the power consumption and the size of optical modules for communications employing a laser diode device (or LD device), as well as to increase the speed of the modulating signal (more than 10 GHz).
Because the faster speed of the modulating signal handled by an optical module includes the more high frequency components in the modulating signal, the transmission lines etc. within the optical module must be impedance matching well for high frequency signals.
Therefore, conventional optical modules are configured such that: a matching resistance is provided between the LD device and the driver circuit for driving the LD device; and the combined impedance of the matching resistance and the LD device is set equal to the impedance of the driver circuit. For example, if the impedance of the driver circuit is 50 Ohms and the resistance value of the LD device is approximately 6 Ohms, then the matching resistance must be set to 44 Ohms. With the increasing speed of the modulating signal, it is advantageous and common practice that the transmission lines between the driver circuit and the LD device are designed to transmit differential signals to the LD device in order to achieve a stable electrical connection.
To reduce the power consumption of the optical module, it is preferable to reduce the matching resistance. However, reducing the matching resistance leads to an impedance mismatch. To compensate for this, a technique for impedance matching is proposed in which a low impedance line having a length approximately equal to ¼ wavelength is provided within the optical module (see, e.g., Japanese Patent Laid-Open No. 2004-259880 (FIG. 5)).
Another proposed-technique for impedance matching is applied to conventional optical modules employing an electroabsorption (EA) modulator. According to this technique, an open stub is connected to a microstrip line constituting a transmission line to form a -matching circuit (see, e.g., Japanese Patent Laid-Open No. 11-38372 (FIG. 1)).
Conventional optical modules including a matching resistance have a problem in that the value of the matching resistance must be increased as the reference impedance to be matched increases. Therefore, an increase in the matching resistance results in an increase in the power consumption of the device, because the LD device is current driven device.
In the case of the above conventional optical module in which the value of the matching resistance is reduced, on the other hand, it is necessary to add a low impedance line having a length approximately equal to ¼ wavelength. This increases the parts count and hence cost, and makes it difficult to reduce the size of the optical module.
Further, the above conventional optical module in which an open stub is connected to a microstrip line to form a matching circuit is intended for use at frequencies of 40 GHz and higher and is disadvantageous in that for the open stub to provide impedance matching at frequencies of 10 GHz and lower, it is necessary to increase the size of the substrate for the matching circuit. This increases the size of the circuit and parts cost, resulting in an increase in the cost of the entire optical module.